Circadian rhythms and environmental lighting regulate a number of endocrine and behavioral functions. Dispersed chick pineal cells remain rhythmic and responsive to in culture. Norepinephrine mimics the acute inhibitory effect, but not the phase shifting effect, of light and can prevent damping of the melatonin rhythm Norepinephrine and light regulate coupling of the pacemaker to melatonin synthesis. The G-proteins, Gs-alpha and its beta- gamma subunits are found in the cytoplasm as well as in the membranes and may translocate in response to stimulation. Adenylyl cyclase activity appears to be rhythmic. Vasoactive intestinal peptide stimulates melatonin synthesis but does not show alpha- l-adrenergic potentiation. Changes in cell volume induced by hypertonic solutions mimic the effects of light on the underlying pacemaker.